Concrete pipes, which are normally used for the discharge of wastewater via sewers as well as for the discharge of rainwater, are generally manufactured such that the pipe exclusively consists of the base concrete which is used for the casting of the pipe.
Since the development within the field of environment and wastewater has involved the separation of wastewater and rainwater, rainwater by-passes the cleaning systems passing through separate conduits to reduce the load on the wastewater cleaning systems, a concentration of the wastewater increases at the same time, which means that the wastewater is even more corrosive for the concrete pipes than before.
To prevent the corrosive wastewater from attacking the concrete pipes, it has been attempted to line the pipes with a material other than concrete for the manufacture of what is called lined pipes. Such a lining may be carried out in several different ways, but generally it is done in that the lining is placed on the core in the mould, following which the concrete pipe is cast and removed from the mould in a conventional manner, which means that the cast pipe and the outer form are lifted off the core, and the outer form is lifted off the pipe.
In addition to increased manufacturing costs and problems of density when joining the pipes to a conduit, this also involves a reduction of the capacity in the manufacture of such pipes. U.S. Pat. No. 5,051,223 discloses another method of manufacturing corrosion resistant concrete pipes. Here, use is made of a radial pressing head which is basically composed of a plurality of oppositely running rolls which form the internal part of a concrete pipe. The external part is formed in an ordinary manner by means of an outer form.
Concrete is poured into the mould, while the radial pressing head is run up through the vertically positioned outer form. When the radial pressing head with its oppositely running rolls runs up through the outer form, these oppositely running rolls form the inner surface of the pipe. A smoothing device is provided at a fixed distance after the oppositely running rolls, seen in the direction of travel of the radial pressing head, an impregnation liquid supplied to the internal surface of the concrete pipe by the smoothing device. This impregnation liquid is supplied while the concrete is still wet, whereby the impregnation liquid in the subsequent smoothing permeates about 10 mm into the inner surface of the pipe wall. This subsequent smoothing, also called polishing, takes place after the pipe has been formed.
Today, however, a large number of concrete pipes are manufactured according to a different method, viz. by the so-called vibration method. For this manufacturing method, it is not possible to use the above-mentioned known technique for the manufacture of corrosion resistant concrete pipes.
In the manufacture of concrete pipes according to the vibration casting method, the technique may again be divided into two methods, viz. the method with a rising core and the method with a fixed core.
In the rising core method, as shown for example in European Patent No. 0 674 573 B, the casting machine comprises a casting mould which basically consists of an outer formwork in the form of an outer form, an inner formwork in the form of a core, a bottom formwork in the form of a bottom ring and a feed system. In addition, there is a profile ring for the forming of the spigot end of the pipe, defining the upward boundary of the casting mould.
In the making of the pipe, vibration is applied via vibration means which may be arranged in the core, on the outer form or as table vibration, where the core or outer form stands on a vibrating table.
In the manufacture of a pipe according to the rising core method, the casting process basically takes place in that the core is moved up through the bottom ring, following which it is stopped, the core being present at a certain height in the mould. Then, concrete is poured into the mould from its upper part. Subsequently, the core is moved continuously up through the mould while concrete is being filled into the mould. The concrete is distributed and compacted during the process by a rotor arranged on the top of the core, while the concrete is vibrated under the action of a strong vibrator, normally arranged centrally in the core. A profile ring is arranged at the upper end of the outer form, and when the mould is filled with concrete, this profile ring defines the upper boundary of the mould, together with the outer form and the core. When the mould has been filled with concrete, pressing is performed, and then the profile ring is rotated slightly to and fro in order to smooth the spigot end. One of the effects of this casting process is that it ensures that the manufactured pipes have the same length.
In the manufacture of pipes according to the fixed core method, the casting process basically takes place in that the core is fixedly positioned, and that an outer form having a bottom ring clamped thereto is arranged over the core. The concrete is then poured down into the mould from above, while vibration is applied to the mould. When the mould has been filled, a profile ring is moved down at the top of the mould, and thereby the spigot end of the pipe is formed. Then the core is removed from the mould by means of a crane or similar lifting tool which positions the pipe, bottom ring and outer form for setting at a suitable location, where the outer form is loosened from the bottom ring and the crane lifts the outer form off and away.
Accordingly, the object of the invention is to provide a method of manufacturing concrete pipes with an inner surface, where the properties are improved with respect to surface quality, e.g. in that the inner surface exhibits a greater corrosion resistance, better flow properties and a more attractive surface structure.
A further object of the invention is to provide an apparatus for the manufacture of such pipes.
The method of the invention, ensures that when the pipe itself has been formed, a further layer of greater structural density is applied to the inner side of the pipe through the inner mould part, the so-called core, via nozzles or gaps, while the mould and thereby the concrete are vibrated. Vibration of the mould keeps it in a form of a fluid phase. When the concrete is maintained in such a fluid phase during the feeding of material of greater structural density, the effect is achieved that the two materials merge to a mutually denser structure, thereby providing a tighter bond between the two materials.
An advantageous way of feeding the material of greater density in structure, is by feeding the further material through the inner mould part, the core, while it is displaced into the outer mould part, feed openings being provided in the core along its circumference at the upper end of the core for the supply of the further material.
A method is described where the applicator in the apparatus is formed by a core which is rotated after the forming of the pipe, a further layer being applied internally to the pipe from one or more rows of nozzles. In this case, the apparatus for the performance of the method is preferably of the fixed core type.
A method is described where the applicator for the application of the inner layer is formed by the rotor, which is also used for forming the pipe beforehand. At its lower part, the rotor is provided with one or more supply openings for the further material.
A method is described where the inner layer is applied to a top and/or bottom ring before these are applied to the other mould parts. This ensures that the parts of the finished concrete pipe which will constitute the parts to be joined in the laying of a pipeline, also have a surface of greater density than the outer surface of the concrete pipe.
A similar method is also described but where the further layer is applied to the bottom and/or top ring after the bottom and/or top ring has been connected with the other mould parts, and before the mould is filled with concrete.
An embodiment of the method is also described where the spigot end of the pipe is provided with the further material to form the inner layer of greater structural density.
The further material may be fed in the form of a paste, powder or liquid.
An apparatus is defined for the performance of the method, where the core, according to the rising core principle, constitutes the applicator. The further material is fed through one or more annular grooves, said grooves extending along the circumference of the core and being arranged at the front end of the core seen in the direction of travel of the core.
An apparatus is also defined with an applicator for a core according to the rising core principle, where the annular groove or grooves are formed with a plurality of nozzles or gaps for the supply of the further material.
An apparatus is also defined with an applicator according to the fixed core principle, where the core is rotated after or during the forming of the pipe. The core is provided with one or more grooves which extend in the longitudinal direction of the core, said groove or grooves extending in the entire effective length of the core. The further material is fed though the groove or grooves adapted for the purpose, while the core is rotated about its longitudinal axis. The core is rotated at least enough so that the entire inner surface of the pipe is covered by the further material.
The apparatus as described above may have the groove or grooves extend as straight grooves in the longitudinal direction of the core.
The apparatus may have the groove or grooves extend in a form of a spiral along the surface of the core in the longitudinal direction.
An apparatus is further defined where the applicator is placed on a rotor in the lower part thereof, so that the further material is fed to the inner surface of the pipe by means of a plurality of nozzles or gaps, just after the pipe itself has been formed.
The further material is preferably fed under pressure and while the mould and/or pipe are vibrated. The pressure may be provided by the supply of the further material, or may optionally be applied by an expedient configuration of the core in connection with the provided gap or gaps in the surface of the core.